A variable nozzle turbocharger generally comprises a center housing having a turbine housing attached at one end, and a compressor housing attached at an opposite end. A shaft is rotatably disposed within a bearing assembly contained within the center housing. A turbine or turbine wheel is attached to one shaft end and is carried within the turbine housing, and a compressor impeller is attached to an opposite shaft end and is carried within the compressor housing.
FIG. 1 illustrates a part of a known variable nozzle turbocharger 10 including the turbine housing 12 and the center housing 32. The turbine housing 12 has an exhaust gas inlet (not shown) for receiving an exhaust gas stream and an exhaust gas outlet 16 for directing exhaust gas to the exhaust system of the engine. A volute 14 connects the exhaust inlet and a nozzle which is defined between an insert 18 and a nozzle ring 28. The insert 18 forms an outer nozzle wall and is attached to the center housing 32 such that it is incorporated in the turbine housing 12 adjacent the volute 14. The nozzle ring 28 acts as an inner nozzle wall and is fitted into the insert 18. A turbine wheel 30 is carried within the exhaust gas outlet 16 of the turbine housing 12. Exhaust gas, or other high energy gas supplying the turbocharger 10, enters the turbine wheel 30 through the exhaust gas inlet and is distributed through the volute 14 in the turbine housing 12 for substantially radial entry into the turbine wheel 30 through the circumferential nozzle defined by the insert 18 and the nozzle ring 28.
Multiple vanes 20 are mounted to the nozzle ring 28 using vane pins 22 that project perpendicularly outwardly from the vanes 20. Each vane pin 22 is attached to a Vane arm 24, and the vane arms 24 are received in a rotatably mounted unison ring 28. An actuator assembly is connected with the unison ring 26 and is configured to rotate the unison ring 26 in one direction or the other as necessary to move the vanes 20 radially, with respect to an axis of rotation of the turbine wheel 30, outwardly or inwardly to respectively increase or decrease the pressure differential and to modify the flow of exhaust gas through the turbine wheel 30. As the unison ring 26 is rotated, the vane arms 24 are caused to move, and the movement of the vane arms 24 causes the vanes 20 to pivot via rotation of the vane pins 24 and open or close a throat area of the nozzle depending on the rotational direction of the unison ring 26.
An example of a known turbocharger employing such a variable nozzle assembly is disclosed in WO 2004/022926 A.
The vanes are generally designed having an airfoil shape that is configured to both provide a complementary fit with adjacent vanes when placed in a closed position, and to provide for the passage of exhaust gas within the turbine housing to the turbine wheel when placed in an open position. Such a vane has a leading edge or nose having a first radius of curvature and a trailing edge or tail having a substantially smaller second radius of curvature connected by an inner airfoil surface on an inner side of the vane and an outer airfoil surface on an outer side of the vane. In this vane design, the outer airfoil surface is convex in shape, while the inner airfoil surface is convex in shape at the leading edge and concave in shape towards the trailing edge. The inner and outer airfoil surfaces are defined by a substantially continuous curve which complement each other. As used herein, the vane surfaces are characterized as “concave” or “convex” relative to the interior (not the exterior) of the vane. The asymmetric shape of such a vane results in a curved centerline, which is also commonly referred to as the camberline of the vane. The camberline is the line that runs through the midpoints between the vane inner and outer airfoil surfaces between the leading and trailing edges of the vane. Its meaning is well understood by those skilled in the relevant technical field. Because this vane has a curved camberline, it is a “cambered” vane.
The use of such cambered vanes in variable nozzle turbochargers has resulted in some improvement in aerodynamic effects within the turbine housing. Some particularly useful vane designs are disclosed in U.S. Pat. No. 6,709,232 B1. These vane designs reduce unwanted aerodynamic effects within the turbine housing by maintaining a constant rate of exhaust gas acceleration as exhaust gas is passed thereover, thereby reducing unwanted back-pressure within the turbine housing which is known to contribute to losses in turbocharger and turbocharged engine operating efficiencies.
Although the use of cambered vanes has resulted in some improvements in efficiency, it has been discovered that there is a risk to get a reversion of aerodynamic torque acting on the vane surface. In particular, it has been observed that there is usually a negative torque when the nozzle throat area is small and that there is a positive torque when the nozzle throat area is large. The torque is defined as positive when the flow of exhaust gas has enough force to urge the vanes into the open position. The aerodynamic torque reversion affects the functionality of the actuator assembly and the unison ring which cause the vanes to pivot. Having regard to controllability, it is preferable that the torque exercised on the vane has always the same orientation regardless of the vane position. It is even more preferable that the torque is positive and tends to open the nozzle (i.e. increase the throat area of the nozzle).